Structure Design And Performance Analysis Of Novel Microwave Power Silicon Carbide MESFET

Silicon carbide (SiC) has found wide application in the fields of high-frequency, high-temperature, high-power and radio-resistant due to its excellent properties such wide gap, high electron saturation drift velocity, high critical electric field and high thermal conductivity. With the rapid development of wireless communication, there has been increasing demand for hardware with high power-density and fast frequency-response. SiC based metal-semiconductor field-effect transistors (MESFETs) have superior advantages over Si and GaAs based devices, being a suitable candidate for a wide range of commercial and military applications, including aerospace, microwave communication, electronic countermeasure, large capacity information processing, etc. In view of the domestic and overseas research status of MESFETs, this dissertation reveals a systemic investigation of the structure design, numerical simulation, reliability and practical application of the SiC MESFETs. The major studies and conclusive results are as follows.(1) The characteristics of4H-SiC MESFETs are analyzed through working mechanism. Suitable device models are built and simulations have been performed using ISE-TCAD. Also, the characteristics dependences on the key structure parameters are discussed.(2) The high density of interface states at SiC/oxide interfaces causes gate-lag phenomena in high-frequency operations, leading to the deteriorated performance. A novel structure of4H-SiC MESFETs is proposed focusing on surface trap suppression, and a device with a0.6μm gate length is investigated based on improved trap models. A P-type spacer layer is shown to suppress surface trap effect and reduce gate-drain capacitance under large drain voltage in microwave operation. The P-type spacer layer incorporated with a field-plate improves the electric field distribution on the gate edge while inducing less gate-drain capacitance than the field-plated structure. The maximum saturation current density of460mA/mm is yielded. Also, the gate-lag ratio under drain voltage of20V is close to90%. In addition, a5%and a17.8%improvement in ft and fmax are obtained compare with buried-gate and field-plated MESFET in AC simulation.(3) Based on the theory of electric field modulation and the analysis of the disadvantages of the field-plated devices, the model of4H-SiC MESFET with a p-type surface epi-layer between the gate and the drain is established considering carrier velocity saturation and impact ionization. The improvement in distribution of the electric field is discussed and the output current (Ids) and breakdown voltage (VB) dependences on the dimensions of the p-type epi-layer are studied using abrupt junction approximation. The optimized design is obtained and the results show that VB is greatly increased by33%with Ids unchanged (less than3%) when the thickness and the doping concentration of the surface epi-layer are chosen as0.12μm and5×1015cm-3, respectively. This approach presents a solution to the constraint condition between Ids and VB.(4) In order to improve the frequency characteristics, the gate-buffer approach is applied in SiC MESFETs. With the incorporated approach, the performance of the device is totally enhanced. A p-type spacer layer, inserted between the oxide and the channel, is shown to suppress surface trap effect and improve the distribution of electric field at the gate edge. Meanwhile, a light-doped n-type buffer layer under the gate reduces the depletion in the channel, resulting in an increase in the output current and a reduction in the gate-capacitance. The structure parameter dependences of the device performance are discussed and the optimized design is obtained. The results show the maximum saturation current density of325mA/mm is yielded compared with182mA/mm for conventional MESFET on condition that the breakdown voltage of the proposed MESFET is larger than that of the conventional MESFET, leading to an increase of79%in output power density. Also, improvements of27%cut-off frequency and28%maximum oscillation frequency are achieved compared with the conventional MESFET, respectively.(5) Based on the analyses of the issue in short-channel devices, an improved hetero-material-gate (HMG) structure is proposed for deep sub-micron SiC MESFETs. Considering the physical models of Schottky barrier lowering and barrier tunneling, the effects of the HMG approach on the channel potential, pinch-off voltage and electric field distribution under the gate are analyzed. It is shown that the HMG approach induces a multi-stepped distribution of the channel potential, leading to an enhanced electric field at the source. Meanwhile, the position of the maximum of the channel potential is changed to the drain side compared with the dual-material-gate (DMG) device, resulting in a better restraint on short-channel effect. Also, different technological parameters and asymmetric gate structures are designed to study the dependences of the device performance, achieving a decreased sub-threshold swing an enhanced VB of the small scale device.In summary, several novel structures of SiC MESFETs are proposed and the characterizations are performed in this dissertation. In-depth theoretical analyses are taken through device modeling and numerical simulation. A number of meaningful and instructive results have been obtained for the design and fabrication of SiC MESFETs.